m4: Extensions
16.1 Extensions in GNU M4
=========================
This version of 'm4' contains a few facilities that do not exist in
System V 'm4'. These extra facilities are all suppressed by using the
'-G' command line option (⇒Invoking m4 Limits control.), unless
overridden by other command line options.
* In the '$N' notation for macro arguments, N can contain several
digits, while the System V 'm4' only accepts one digit. This
allows macros in GNU 'm4' to take any number of arguments, and not
only nine (⇒Arguments).
This means that 'define(`foo', `$11')' is ambiguous between
implementations. To portably choose between grabbing the first
parameter and appending 1 to the expansion, or grabbing the
eleventh parameter, you can do the following:
define(`a1', `A1')
=>
dnl First argument, concatenated with 1
define(`_1', `$1')define(`first1', `_1($@)1')
=>
dnl Eleventh argument, portable
define(`_9', `$9')define(`eleventh', `_9(shift(shift($@)))')
=>
dnl Eleventh argument, GNU style
define(`Eleventh', `$11')
=>
first1(`a', `b', `c', `d', `e', `f', `g', `h', `i', `j', `k')
=>A1
eleventh(`a', `b', `c', `d', `e', `f', `g', `h', `i', `j', `k')
=>k
Eleventh(`a', `b', `c', `d', `e', `f', `g', `h', `i', `j', `k')
=>k
Also see the 'argn' macro (⇒Shift).
* The 'divert' (⇒Divert) macro can manage more than 9
diversions. GNU 'm4' treats all positive numbers as valid
diversions, rather than discarding diversions greater than 9.
* Files included with 'include' and 'sinclude' are sought in a user
specified search path, if they are not found in the working
directory. The search path is specified by the '-I' option and the
'M4PATH' environment variable (⇒Search Path).
* Arguments to 'undivert' can be non-numeric, in which case the named
file will be included uninterpreted in the output (⇒
Undivert).
* Formatted output is supported through the 'format' builtin, which
is modeled after the C library function 'printf' (⇒Format).
* Searches and text substitution through basic regular expressions
are supported by the 'regexp' (⇒Regexp) and 'patsubst'
(⇒Patsubst) builtins. Some BSD implementations use extended
regular expressions instead.
* The output of shell commands can be read into 'm4' with 'esyscmd'
(⇒Esyscmd).
* There is indirect access to any builtin macro with 'builtin' (⇒
Builtin).
* Macros can be called indirectly through 'indir' (⇒Indir).
* The name of the program, the current input file, and the current
input line number are accessible through the builtins
'__program__', '__file__', and '__line__' (⇒Location).
* The format of the output from 'dumpdef' and macro tracing can be
controlled with 'debugmode' (⇒Debug Levels).
* The destination of trace and debug output can be controlled with
'debugfile' (⇒Debug Output).
* The 'maketemp' (⇒Mkstemp) macro behaves like 'mkstemp',
creating a new file with a unique name on every invocation, rather
than following the insecure behavior of replacing the trailing 'X'
characters with the 'm4' process id.
* POSIX only requires support for the command line options '-s',
'-D', and '-U', so all other options accepted by GNU M4 are
extensions. ⇒Invoking m4, for a description of these
options.
The debugging and tracing facilities in GNU 'm4' are much more
extensive than in most other versions of 'm4'.
Info Catalog
m4: Compatibility
m4: Incompatibilities